PURPOSE: Although defecatory function after low anterior resection for rectal cancer is reported to be better following colonic J-pouch than straight anastomosis, few prospective randomized trials comparing the two forms of anastomosis have been reported. We performed a prospective randomized trial comparing straight anastomosis with colonic J-pouch anastomosis both clinically and physiologically in patients undergoing stapled low colorectal anastomosis. METHODS: A total of 42 consecutive patients were intraoperatively randomized to undergo either straight anastomosis or colonic J-pouch anastomosis. Clinical defecatory function was evaluated by a questionnaire answered preoperatively, then 6 and 12 months postoperatively. Anorectal physiological assessment was also carried out before surgery, then 12 months postoperatively. RESULTS: The clinical defecatory function assessed 6 months and 12 months after surgery did not differ between the two groups. However, while the length of high-pressure zone was significantly shortened, and (neo)rectal capacity was significantly reduced postoperatively in the straight group, none of these physiological parameters were significantly altered in the pouch group. CONCLUSION: Although the aim of colonic J-pouch to preserve reservoir function was physiologically achieved, the improvement in clinical defecatory function was not significant. Thus, further prospective studies are needed to confirm the functional superiority of colonic J-pouch anastomosis for stapled low colorectal anastomosis after low anterior resection.